1. Field of Invention
This invention generally relates to electronic measurement, specifically to the measurement of radiated waves that can be used for communication or can interfere with communication, to the calibration of equipment that might be used to perform those measurements or other tasks.
2. Prior Art
Communication Measurement
There are many circumstances in which it is important to measure the strength of radiated signal accurately. Two examples occur when it is desired to determine why an existing communication installation is not performing adequately and when it is desired to design a communication installation. In some branches of the communication industry this measurement process is called a “site survey” and that nomenclature will be applied here for all measurements made before equipment is finally installed. When the goal of the site survey is to analyze an existing installation, it is usually to determine why the existing installation is not transferring data at a desired rate from one location to another. An inadequately functioning installation often needs to be upgraded. Designing of an upgrade to the existing installation is similar to a second goal, designing a new communication installation; these requires predicting the rate at which error free data will be transferred (throughput) in the new or modified installation. The techniques and apparatuses presented here allow installation design to be done with much greater accuracy than with the prior art. Consequently, the resulting designs are significantly more effective and cost efficient.
Prior Art Methods
Previously, the same equipment type (make and model) that will be installed, was used to perform the site survey. Occasionally, more than one equipment type was used requiring an obvious duplication of the following procedures. In the following procedures the installation is viewed as comprised of a network of directed simplex links which are measured individually. Several alternative procedures are in use. They follow the same pattern as shown in FIG. 1 with salient differences in steps 5 and 6:                1. Determine what specifications are to be met by the installation and derive the requirements of the links needed to meet them (100).        2. Select what equipment will be used at each end of the proposed link. The equipment at opposite ends of the link may be different (102).        3. Propose a transmitter antenna location and receiver antenna location (104).        4. Place one sample of each selected antenna at each end of the measured link. Place the associated samples of the selected equipment close enough to be connected (106).        5. Use the equipment to estimate signal properties (108 selects 110 or 112).        6. If the requirements are not met, the configuration must be changed (often equipment location or antenna type) and steps 3 through 5 are performed again (114).        7. The equipment is moved successively to all of the locations where links are needed and steps 3 through 6 repeated (116).        
A generic problem with the basic procedure comes from the determination of what equipment will be used (102}. It requires that the equipment to be used, be selected too early in the design cycle. In most cases, there is some latitude in equipment selection. This latitude is discarded often before the designer has even seen the location where the equipment is to be installed and clearly before any site survey has been made. It is precisely the results from the site survey that are needed to select the equipment that is best suited to a specific installation. In practice, even if the designer of an installation wanted to change his equipment choice once he has made the site survey, he could not. Not only would he have to discard the data from the site survey, but typically he does not have a sample of the alternative equipment with him.
Prior Art Procedure 1
A common means of implementing a site survey is to use software to generate a potentially large amount of data traffic to be sent over the communication path and to measure the amount that actually is transferred in step 5 (112). This is a saturated throughput measurement, and is the method used in U.S. Pat. Nos. 6,442,507 and 7,162,507. The requirement is stated in terms of a requisite saturated throughput and if the saturated throughput is sufficient to meet the requirement, then step 6 succeeds (114). This will be referred to as prior art procedure 1.
A problem with procedure 1 comes from errors in the site survey measurement. The procedure above can determine whether a particular configuration will work with the samples of equipment used in the site survey, but not the equipment that will ultimately be installed. If the equipment ultimately installed is in some sense weaker (usually lower output signal strength or poorer sensitivity) than the equipment used for the site survey then the measurement does not necessarily mean that the ultimate configuration will provide the requisite saturated throughput.
Prior Art Procedure 2
A solution to this second problem is theoretically available on installations that use multi-rate protocols. This will be referred to as prior art procedure 2. In it, saturated throughput is measured in step 5 (112). It is compared with a requirement that is a greater saturated throughput than is actually needed in the final installation. This procedure does indeed provide some margin, but it is difficult to know how much. The relationship between signal strength and saturated throughput is complex and often requires significant additional effort to estimate. Furthermore, this relationship is altered significantly by the presence of interference. This method appears to guarantee little more than a change in the right direction and will not be analyzed further. Other variations of this method are possible, but the author is aware of no one using such methods.
Prior Art Procedure 3
A second measurement option is available on installations that use equipment that provide a mechanism to measure the signal strength received (often called a “Received Signal Strength Indicator” or RSSI) and provide it to the user. This is a signal strength measurement, and is the method recommended by Lisa Phifer (“How to Install a WLAN” By Yuval Shavit. 15 Feb. 2008. SearchNetworkingChannel.com.) This will be referred to as prior art procedure 3. In it, RSSI is measured in step 5 (110). The requirement to be met in step 6 is the manufacturer's specification of the signal strength required by the receiver. There are several problems with this procedure, the paucity of equipment that provide RSSI data, the accuracy of the RSSI data when provided, inability to measure or compensate for interference to the communicating signal, and the problem with weaker equipment incurred by prior art procedure 1.
The purpose for which RSSI data is provided to the user is to allow the user to select the most suitable signal or to know if a signal is strong enough for his use. This function is useful but not necessary in client nodes which sometimes are used in point-to-multi-point communications and it is often not provided. Additionally, much equipment is designed for applications where no user is present. As a result there is often no reason to make RSSI data available to a user and it is not done. If RSSI data is made available to the user, it is often made available with only very course resolution, perhaps roughly half a dozen levels. This is sufficient for the purpose for which it was intended, but far too little resolution to measure signal strength in a meaningful way. As a result, the need to use RSSI data restricts the choices of equipment that can be considered for a specific installation to that equipment with the RSSI measurement mechanism. This restriction can be quite serious. The lack of a measurement mechanism is so prevalent in some categories of point-to-multipoint equipment that the measurement is simply not made on the half of the links connecting to the infrastructure node (sometimes called an “Access Point.”) This partially defeats the purpose of performing a site survey.
The use of RSSI data also introduces additional error in the site survey. In those instances where RSSI data is provided, its accuracy is often unspecified. In equipment that the author has examined, RSSI accuracy is much better at high signal strength than at the low signal strength used to determine whether a configuration will work.
While saturated throughput measurements inherently include the effect of interference on the throughput, signal strength measurements do not include many types of interference. Interference can reduce throughput even to zero without affecting the RSSI.
In a manner similar to in prior art procedure 1, the procedure above may determine whether the signal strength is adequate in a particular configuration with the samples of equipment used in the site survey, but not the equipment that will ultimately be installed.
Prior Art Procedure 4
Equipment that provides RSSI data to the user also makes it possible to overcome the problem with weaker equipment. As in prior art procedure 3, RSSI is measured in step 5 (110). In step 6 the requirement is set at some margin above the manufacturer's specification of the signal strength required by the receiver to overcome the difference in signal strength that may exist between the equipment used for the site survey and that finally installed. This will be referred to as prior art procedure 4. It incurs all of the other problems of prior art procedure 3.
Prior Art Error Analysis
To put prior art errors in perspective, error information for equipment operating in the low-gigahertz frequency range will be presented. The primary contributors to the overall error are:
transmitter output signal strength tolerance;
receiver sensitivity tolerance;
variation in the peak gain of antennas; and
antenna gain variations with direction.
The tolerances specified by most manufacturers are in dB around a nominal specified value (e.g. a transmitter may have an output power of 36 decibels relative to a milliwatt+ or −2 decibels, written 36 dBm+/−2 dB or 36+/−2 dBm.) Such specifications indicate that the weakest component can be two times the tolerance less than the strongest (in the example given the weakest transmitter will have an output power of 34 dBm and the strongest, 38 dBm.); thus, a difference of 4 dB can exist between the weakest and strongest transmitters that meet this specification. This difference shows up as an error in site survey measurements: thus, measurements have a property which is their error, expressed as the difference between the strongest and weakest extremes unless otherwise noted.
The significance of these errors can be better understood if their origin is known. Transmitter and receiver errors come from many sources including:
component variation;
component aging (particularly microwave semiconductor components);
variation with the frequency being used; and
variation with temperature.
Most vendors do not publish openly the difference between the strongest and weakest signal from the transmitter and the difference between the sensitivity of the best and worst-case receiver in a model of a product, though some do. Often the difference can be obtained from the vendor if requested (as the author has done), but these specifications may not be binding on the vendor and some units may not lie within the limits given. None-the-less, in most cases each difference given is no smaller than 4 dB (often expressed as +/−2 dB.)
The gain of commodity antennas is usually specified loosely, often by a single number. Antenna-to-antenna variation (tolerance) within the same model is usually not specified, but unofficially manufacturers are reluctant to indicate a consistency greater than +/−0.25 to +/−0.5 dB, partially due to the difficulty in measuring gain. Based upon interviews with experts, the author expects that most antenna designs would be consistent within +/−0.25 dB (with perfect measurement capability) and some technologies will deliver somewhat better consistency. For the purpose of this paper, 0.5 dB (from +/−0.25 dB) will be used as the best to worst antenna gain ratio.
Omnidirectional antennas are used in most cases where point-to-multi-point communications is desired anywhere within a service area. These antennas have an additional source of error called “omnidirectional asymmetry.” Omnidirectional asymmetry is the degree to which an omnidirectional antenna gain deviates from uniformity in all directions in the plane of omnidirectionality (usually the H-plane.) It can easily be observed by rotating the antenna relative to the point of observation, and this data is published by some manufacturers. The author found the average omnidirectional asymmetry of indoor antennas to be about 2.5 dB. There was not enough data to bound the error, but there was enough data of identify the standard deviation of omnidirectional asymmetry as about 1.5 dB. Omnidirectional asymmetry is important because in most cases it is not possible to guarantee that the orientation of the antenna as finally installed is identical to the orientation of the antenna used in the site survey. Indeed, in some cases omnidirectional asymmetry is dictated by subtle alignments hidden inside a random.
Outdoor antennas have typically have less error, averaging about 1.5 dB, but there was not enough data to compute a reliable standard deviation. Outdoor antennas must also contend with variations in the attenuation of the coaxial cable and connectors used to connect the antennas to the electronics interfacing with them. This is significant for long cables.
For the purpose of this analysis, one standard deviation worse than the average indoor antenna omnidirectional asymmetry (4 dB) will be used for the contribution to the overall error due to omnidirectional asymmetry. With some degree of care it can be achieved, though the author is not aware of anyone exercising that degree of care.
Prior art procedures 3 and 4 make use of RSSI: RSSI data itself contains errors as discussed in Receiver Signal Metrology. Sometimes the amount of additional signal strength required by prior art procedures 4 is sufficient to raise the signal strength out of the levels with the highest error. For the purpose of this example, a 4 dB error will be used, but prior art procedure 3 is less likely to gain the same benefit.
Antenna gain change over the band being used is another source of error in measurement and should be included in this analysis. It is not for two reasons. First, the range of frequencies being used is highly dependent on the application, and second, manufacturers usually supply only band-center data.
The aggregate errors that are possible under these conditions can be summarized as follows:
4.0 dB transmitter output signal strength error;
0.5 dB peak gain of the transmitter antenna error;
4.0 dB omnidirectional asymmetry of transmitter;
0.5 dB peak gain of the receiver antenna error;
4.0 dB omnidirectional asymmetry of receiver;
4.0 dB receiver sensitivity error;
4-7 dB RSSI error prior art procedure 3; and
4.0 dB RSSI error prior art procedure 4.
The total error for prior art procedure 1, point-to-point communication (no omnidirectional asymmetry) is about 9 dB of error. Additional error is incurred in the following circumstances:
8 dB for point-to-multi-point communication;
Incalculable for prior art procedure 2;
4-7 dB for prior art procedure 3; and
4.0 dB for prior art procedure 4.
The most frequently encountered error is that of prior art procedure 4 implemented for point-to-multi-point communication. That is a 21 dB error. The error bounds given here are quite accurate for a set of circumstances with which the author is familiar and illustrative of many more. They allow the consequences of site survey errors to be examined.Consequences of Errors
The signal strengths, gains, sensitivities etc. of any of the equipment used to perform a site survey are usually not known more accurately than the manufacturer's specifications. To understand the implication of that, the bound upon these properties will be explored. Assume that the equipment and configuration used to take the site survey was all at the maximum end of its tolerances and that the equipment as installed and configured was at the minimum. The results from the above analysis show that a disparity of from 9 to 24 dB exists. If prior art procedure 1 is used, no correction for this disparity will be applied, the installation can have signal levels 9 to 17 dB too weak to meet requirements. If prior art procedure 3 is used, again no correction will be applied for this disparity, the installation can have signal levels 13 to 24 dB too weak to meet requirements. These are large errors. If prior art procedure 4 is used then the requirement that must be used in step 6 to guarantee that the installation will meet requirement has a 13 to 21 dB margin over the specification.
Furthering the exploration of these properties, assume that the equipment used to take the site survey was all at the minimum end of its tolerances and that the equipment actually installed was at the maximum. The direction of the disparity is now reversed. If prior art procedure 1 or 3 is used, the installation will meet specifications with excess signal strength of 9 to 24. If prior art procedure 4 is used with the requirement that must be used in step 6 to guarantee that the installation will meet specifications, then the signal strength will be 22 to 38 dB greater than is needed to meet specifications.
The consequence of not meeting specifications is obvious, the network does not transfer data at the rate required, and may not transfer data at all in certain locations. The consequence of meeting specifications with such large signal strength excess requires a bit more examination. A point-to-point installation designed by procedure 1 or 3 will have 9 to 13 dB excess signal strength,
A point-to-multipoint installation designed by procedure 1 or 3 will have 17 to 24 dB excess signal strength. These are exactly the margins that should exist to accommodate the possibility that the equipment actually installed could be at the minimum end of its tolerances. However, an installation designed by procedure 4 using equipment at the minimum end of its tolerances for measurement and installing equipment at the maximum end of its tolerances will have 22 to 38 dB excess signal strength, 9 to 17 dB of which should exist to accommodate the possibility that the equipment actually installed could be at the minimum end of its tolerances. The remaining 13 to 21 dB excess signal strength is due to the margin which is inflated by the need to guarantee that the network will meet specifications in spite of the error in signal strength measurement.
In free-space, radiated signals loose strength as the inverse square of the distance traveled (when signal strength is expressed as power): thus, under these conditions a point-to-point network designed by the signal strength procedure with margin would appear to require 4.5 times more equipment to traverse a distance than actually required, and a point-to-multipoint network designed by the signal strength procedure with margin under the same conditions would appear to require 126 times more equipment to service an area than actually required. Someone skilled in the art of designing radio communication installations will may recognize such results as erroneous, but such a site survey is, at best, useless. Installations with large excess signal strength, cost much more than necessary to purchase, install, administer and maintain. For this reason, until recently designers have been reluctant to use such large margins.
(It should be noted that indoor installations often do not closely follow the free-space relationship used above. This reduces the difference between the amount of equipment that appears to be required due to measurement error and the equipment actually required, but the difference is none-the-less significant.)
The most common margin that the author has encountered as a “rule of thumb” is to provide a margin of 10 dB (in the place of the 13 to 21 dB described above) (Jack Unger. Deploying License-Free Wireless Wide-Area Networks. Indianapolis: Cisco Press, 2003. pp. 52 & 244) While it is not sufficient to cover all cases, it's origin is now clear.
Prior art procedures 1, 3 and, with the use of a 10 dB margin, procedure 4 all lead to a common approach in communication design. A site survey is performed, the installation is built, the installation's users identify regions and situations in which the installation does not meet requirement and the designer is called out to the site to design patches to the installation which are installed. (See the comments of Michael Brandenburg and Craig Mathias in Shavit, 2008.) Unger observes that this is an extremely expensive process in Unger. 2003. p. 285. Occasionally, installations have sufficiently stringent requirements that patching is not adequate and portions of the installation must be redesigned and replaced. The cycle has the potential to repeat itself a seemingly unlimited number of times. In many business situations such a delay is unacceptable. There are even designers who are prone to claim after some effort to patch an installation, that wireless communication should never be expected to be reliable.
More recently, designers have used very large margins with procedure 4 or with no measurements at all, sometimes in an effort to guarantee that the installation will meet requirements when first installed (See the comments of Craig Mathias in Shavit, 2008 and “How to Make Your Wireless LAN Work: Design for Context. Coverage & Capacity” By Motorola, February 2006, p. 3.) This can result in installations that are much more expensive than needed. Not surprisingly, these approaches are sometimes used by those selling equipment.
Receiver Signal Metrology
Previously, many receivers, particularly inexpensive receivers, have had no measurement capability built into them. Some had various forms of signal strength measurement. Among those, some reference signal strength to background noise using it as a source of calibration. Others provide some degree of absolute received signal strength measurement (often called a “Received Signal Strength Indicator” or RSSI.) RSSI data may contain significant errors. The limited experience of the author suggests that the data can have surprisingly good linearity at high signal levels. Based on that experience, it appears that RSSI data can contain at least 34 dB of error at high amplitude increasing to about 7 dB at low amplitude. This amount of error is a significant impediment to making measurements accurate enough to limit the consequences of measurement error in the cost of communications installations.
Weak Signal Generation
Previously, weak signals were generated by attenuating the signal from relatively strong signal sources to achieve the desired signal strength. The accuracy of the resultant signal was the result of all the inaccuracies of all of the components used to generate the signal. Attenuators with large attenuation also have greater errors than those with smaller attenuation. As a consequence, weaker signals have greater uncertainly of signal strength.
An example will show the principle. Assume that the signal source is a low power transmitter. Such a choice is often the cheapest and most readily available signal source. In some cases, it is the only signal source capable of the complex modulation scheme needed by the receiver or it along with the transmitter in a transceiver are the only equipment capable of the complex interactions required by the protocol that the receiver (along with its associated transmitter) require. Many such transmitters have a +/−2 dB power output tolerance as discussed in Communication Measurement, Prior Art Error Analysis. The transmitter will be assumed to have a nominal output power of 26 dBm for the purposes of this example. Midwest Microwave provides good quality commercial attenuators that could be used to create a weak signal from the output of this transmitter in their model ATS-3550-12-NNN-02. Other manufacturers provide similar products. The uncertainties of attenuation below 12.4 GHz. are specified as follows:
AttenuationUncertainty(dB)(dB)100.6201.0Similarly, Agilent 8494 step attenuators provide the ability to attenuate in 1 dB steps with an uncertainty of 1.2 dB or less below 12.4 GHz.
For the purpose of this example, the uncertainties introduced by mismatch among the components will be assumed negligible.
With these components any signal at or below 26 dBm can be generated with some uncertainty. Some samples are as follows:
Signal LevelUncertainty(dBm)(dB)264205.206.2−207.2−408.2−609.2−8010.2−10011.2−12012.2.While this amount of error is acceptable for some applications, it does not allow such applications as calibration of receiver measurements (such as RSSI) as discussed in Receiver Signal Metrology to an accuracy sufficient to limit the consequences of measurement error to the cost of communications installations as discussed in Communication Measurement.